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Structure, magnetic and related properties of the ternary BaAl4 and ThMn12-type compounds
Physica 130B (19851 195-201 North-Holland, Amsterdam
S T R U C T U R E , M A G N E T I C A N D R E L A T E D PROPERTIES OF THE T E R N A R Y B a A i 4 A N D ThMnlz-TYPE C O M P O U N D S Wojciech SUSKI Institute for Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 5 0 - 9 5 0 Wroclaw 2, Poland Invited paper In this review the crystal structure of tetragonal, ternary BaAI4 and ThMn~2 derivatives (14/mmm) is presented. The T h C r : S i , as well as (RE, An)M,AI,, (RE, An)MsAI7 and (RE, An)M~AI~ compounds deserve particular attention. The problem of bonding in these materials is discussed in terms of a possible heterodesmic character. Then the investigations of the crystal field, are briefly reported. Finally, magnetic properties including magnetic structures and Mossbauer eftect examinations are described.
1. Introduction The structure, magnetic and related properties of the tetragonal ternaries with the BaAI4 and ThMn~2 type derivative structure (I4/mmm) arc presented below. These types of structure are represented among numerous compounds of the rare earths and actinides. Particular attention deserved the ThCrzSi2-type ternaries (BaAI4 derivatives) because of their various exciting properties like heavy fermion superconductivity, dense Kondo behavior, valence instabilities and interesting magnetic properties found in these materials. Also the ThMnlz-type derivatives, especially these of the rare earths, exhibit fascinating magnetic properties. Therefore an extensive presentation of these materials seems to be interesting. Both groups of compounds have the same body centered tetragonal structure (14/mmm) and have been already reviewed by Leciejewicz [1] and Nowik and Felner [2]. Thus at present we try complete those papers emphasizing some of their highlights.
2. Crystal structure The tetragonai BaAl4-type structure was originally determined by Andress and Alberti [3] but the first ternary derivative which will be at most a
subject of the present paper - ThCrzSi2 was reported by Ban and Sikirica [4] and independently by Zarechnyuk et al. (for CaAI2Gez) [5]. Recently, this type of structure along with its derivatives were discussed in details by Parth6 et al. [6] and Pearson [7]. More than 200 rare earth (RE) or actinide (An) transition metal (M) borides, gailides, silicides, germanides and phosphides (X) (see [1, 6]) crystallize with the ThCr2Siz-type of structure. The space group is 14/mmm and the c/a ratio in these compounds is usually close to 2.5. The Th atoms are surrounded by 22 atoms (four Th atoms, eight Cr atoms and eight plus two Si atoms) in a polyhedron with a surface composed of 16 triangular and 12 square faces. In this structure eight 4(RE,An)-M, eight I(RE,An)-X and 1X-X interatomic contacts are generally close enough to be of interest in bonding and magnetic interaction problem, whereas other contacts are generally at distances that are so much greater than the appropriate radius sums, as to appear to be of no interest. The similar conclusion concerning a very weak C e - C e bonding (if any) was presented in [8] and this statement was extended to a new series of isotypic compounds {RE, Th, U)(Ru,Os)2Si2. In discussion on interatomic contacts as the cell dimension controlling factors, Pearson [7] observed that the behavior of the Mn-alloys generally differs from that of the
0378-4363/85/$03.30 (c] Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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W. Suski / Properties of the HaAl~ anti ThMnt., type derwattve~
o t h e r transition metals. l"his r e m a r k is i m p o r t a n t in relation to f u r t h e r discussion on m a g n e t i c properties of these materials. Recently, B r a u n ct al. [9] r e p o r t e d p o l y m o r p h i s m of l,alr2Si2 comp o u n d exhibiting a high t e m p e r a t u r e form crystallizing with the primitive, tetragonal C a B c , G c , type of structure ( P 4 / n m m ) . ( ' e r l a i n l v lhe l ' h M n ~ e type of structure and its derivatives are less p o p u l a r than the c o m p o u n d s discussed a b o v e . The most p o p u l a r derivatives arc (RE,An)M,AI,,. I RE,An)MsAI7 and (RE,An)M4AIx. In this b o d y - c e n t e r e d tetragonal crystal structure (14/mmm) the R E / A n a t o m s o c c u p y the 2 (a) sites. In (RE,An)M~,AI,, the (f) positions arc o c c u p i e d by M a t o m s and the (il positions by AI atoms, w h e r e a s the r e m a i n i n g M and AI are distributed at r a n d o m over the (j) sites [2]. The 57Fe M o s s b a u e r studies of REFe~AI7 [2J show that 5 2 - 6 0 % of the iron a t o m s o c c u p y the (f) sites, a b o u t 4(1% the (j) sites and 11-10% the (i) sites. The (RE,AnlM4AI~ c o m p o u n d s form a superstructure of the CeMn4Al~-type in w h i c h M and AI a t o m s are not distributed statistically over the three a t o m i c positions available, but instead the M a t o m s o c c u p y only one (f) site while the AI a t o m s two o t h e r ones ((i) and (j)). Pearson [11] p o i n t e d out that the cell d i m e n s i o n s of the ( R E , A n t M 6 A I , and ( R E , A n l M 4 A I g phases ind i c a t e d relative transition m e t a l sizes in the o r d e r Fe < Cu < Mn < Cr, w h e r e a s in the e l e m e n t a l m e t a l s m a n g a n e s e is larger than c h r o m i u m w h i c h is approximately of the same size as iron. In the e l e m e n t a l m e t a l s the relative sizes u n d o u b t l v result from m a g n e t i c interactions but in the ternaries the o r d e r of sizes suggests an a b s e n c e of s t r o n g m a g n e t i c interactions. This s t a t e m e n t s e e m s to be not entirely valid in the light of the m a g n e t i c properties of these alloys described below.
3. Chemical bonding The p r o b l e m of c h e m i c a l Ix)nding in the tetragonal u r a n i u m ternaries, including the "l'hCr2Si2 type phases has been considered bv Z y g m u t [l(l]. It s e e m s that his main conclusion.,, can be also applied to the rare e a r t h c o m p o u n d s .
For the quantitative discussion hc selected EJPd2Si2 for w h i c h the crystal structure has been precisely refined 1112]. In c o o r d i n a t i o n polyh e d r o n of uranium [Si(UaPd4Si)] onc can distinguish the shortest Si-Si, S i - P d , Si-U and t l-Pd distances, which are e q u a l to 2.38, 2.44 3.13 and 3.24 ,g,, respectively, it is c o n c l u d e d that the lirst and the s e c o n d distances c o r r e s p o n d to sum of the c o v a l e n t radii, and thus to c o v a l e n t b o n d i n g . The Si-U distance from one hand is shorter than simple sum of i o n i c radii 13.68 ]k) but l o n g e r than the sum of c o v a l e n t radii (2.59 /~). Because the Si-atoms is strongly electronegative with respect to u r a n i u m , one can e x p e c t the i o n i c - c o v a l e n t b o n d i n g . Finally, the P d - P d and U - P d b o n d i n g s are c o n s i d e r e d as metallic. T h e r e f o r e , one can a s s u m e heterodesmic c h a r a c t e r of chemical b o n d i n g to exist in the ThCr2Si2 structure. For UFeaAI~ the shortest distances a m o u n t to [13] u - u = 5 . 0 3 6 ,~, U - F c = 3.34O ~ , U-AI = 3 . 0 0 9 ~ , F e - F e = 2.518 A, F e - A I = 2 . 5 3 8 /~ and AIA 1 = 2 . 7 3 0 ~ , but t a k i n g into a c c o u n t the c h a r a c t e r of e l e m e n t s and high electrical conductivity of these materials [14] one can expect metallic c h a r a c t e r of b o n d i n g in the ThM,.3 dcrivativcs. N c m o s h k a l e n k o ct al. [15] suggest also a heterodesmic c h a r a c t e r of bonding {covalent and m e t a l l i c ) i n the REAI2Si2 type family, which in spite of the same stoichiomctry is representative of the hexagonal La2(),S type of structure. For ( ' e ' l ' 2 S i 2 l T = ( ' u , Ag, Au, Pdl c o m p o u n d s it is the hybridization of Cc 4f e l e c t r o n s with "l'-dcrivcd d-states w h i c h is supposed to d e t e r m i n e the g r o u n d state properties (superconductivity for T = ('u. o r d e r e d m a g n e t i s m for 1" = Ag, Au, Pdl. This s t a t e m e n t was c o n c l u d e d from the r e s o n a n t photoemission studies on these systems [ 16]. It was o b s e r v e d that for RERh2Si2 series lanthanide c o n t r a c t i o n manifests itself in the c-lattice c o n s t a n t , w h e r e a s in REPd2Si2 in the a-lattice c o n s t a n t suggesting a fine difference in bonding c h a r a c t e r in these two directions [171.
4. Crystal field This p r o b l e m was not considered up to now in details. No suggestion was published c o n c e r n i n g
W. Suski / Properties of the BaAIa and ThMnl. type derivatives
the ThCrzSi2 type c o m p o u n d s of actinides and the ThMn~2 derivatives from obvious reason of complexity of these materials. For the q'hCr2Si2 type c o m p o u n d s of rare earths no information is available with respect to d electron metals. According to neutron diffraction results d electrons in these materials, except of Mn-compounds, are itinerant and thus it is difficult to discuss thc crystal field ( C E F ) problem. Because the values of rare e a r t h magnetic m o m e n t s are as a rule very close to the Ixng,,/J(J + 1) value, one can conelude that the C E F acting on the rare e a r t h ion is weak. The C E F Hamiltonian for a rare e a r t h ion in a tetragonal point symmetry is given by =
B404
+ B ] ( ) ~ + B . O ~ , + l::J,,06,
but as mentioned above, the general discussion of magnitude of the individual terms and corresponding level schemes is lacking. In individual cases the consideration of magnetic and related properties in terms of the C E F is more detailed. Schlabitz et al. [18] claim that a slight deviation of the magnetic susceptibility of RECu2Si2 (RE = Th, Dy, Ho, Er and Tm) and DyNizSi2 [19] from the C u r i e - W e i s s law below 100 K, is apparently due to the C E F effects. Moreover, they report [18] magnetostriction data of (Y~_~RE~)CuzSi2 which follow the Stevens f a c t o r in sign and to some extent even in magnitude across the RE series, except for Tm, Nd and Sm [20]. Gorlich et al. [19] however, are convinced that the C E F interactions effect considerably the nuclear hyperfine coupling in the dysprosium compounds. Stewart and Zukrowski [21] try to explain their 1 6 9 T m Mossbauer data for TmCuzSi2 in t e r m s of the C E F effects but using the C E F parameters obtained before for CeCuzSi2 by m e a n s of neutron diffraction [22]. For TmCu2Si~ they assume the C E F level scheme with 2 low-lying singlets well separated from the three o t h e r singlets and four n o n - K r a m e r s doublets. This s c h e m e gives a fair fit of experimental results. For CeRh2Si2 the following functions were proposed for the C E F ground-state doublet: 10) = 0.91 + 5/2) + 0.18] + 3/2) [22]. At the same time the large values of the magnetic m o m e n t found in TbNi2Si2 and TbCozSi2 [24], TbRu2Si2
197
[25] and TbRhzSi2 [23] suggest that the C E F of D4h symmetry acting on the T b 3. ion has to p r o d u c e a g r o u n d state with two very close singlets: 1/,/21161 +1 - 6)). In this case the magnetic m o m e n t s are quenched a l o n g the c-axis giving rise to an Ising-like behavior. Some authors claim that the magnetic structure of c o m p o u n d depends on the sign of the B ° term (see [19,211]). In accordance with the previous conclusion that the crystal field in the ThCrzSi2 c o m p o u n d s is weak, an overall splitting in CeCu2Si2 of 31 meV was found by inelastic neutron scattering [22]. Moreover, the anisotropy of the static susceptibility of the YbCu2Si2 single crystals [26] shows that the C E F effects must o v e r c o m e the valence fluctuations. However, the results obtained for the Ce and Yb c o m p o u n d s could be exceptional because the ('e ion has at most 4fI electron, whereas the Yb ion the 4f-~ hole. For YbCu2Si2 in a tetragonal crystal field the J = 7/2 H u n d ' s rule g r o u n d state splits into four doublets. The careful analysis [27] of paramagnetic neutron spectra yields two most likely sets of the C E F parameters namely: 1) W = 2 . 5 + 0 . 3 meV, x~ =-11.41 + 0 . 1 , x2 = + 0 . 0 7 + 1 1 . 0 5 , x3=(1.15+11.115 and x4=(1.13 ± 0 . 0 5 or 2. W = -2.5 +0.3 meV, x, = 0.4 + 0 . 1 , x2 = ± 0.15 + 0.05, x3 = -(1.()7 :i: 0.05, x4 = + 11.13 + 0.05. Both sets have similar energy-level sequence of 11--18-25-31 meV.
5. Magnetic properties A c c o r d i n g to the author's best knowledge, magnetic properties (including magnetic structure and M6ssbauer effect) of more than 120 c o m p o u n d s of rare earths with the ThCr2Si2 type of structure have been examined. A m o n g them about 25 a p p e a r e d to be nonmagnetic and 12 superconducting. At the same time a m o n g 12 actinide compounds (with Th, U and Np) UOszSi2 [28], URu2Si2 [29] and URh2Ge2 [311] exhibit paramagnetic properties. All the ThMn12 type compounds both of the rare earths and actinides, except of NpCu4AIs and one modification of NpCr4AIs, are reported to be magnetically o r d e r e d [31]. As a rule vast
It~8
w. Suski / Properties of the Ba/'iLj and lhMn,., type derit'atires
majority of the ThCr2Si2 type compounds exhibits antiferromagnetic properties with the N6el point below 50 K (see eg. [I]). ()nly for somc terbium compouds the N6el point is close to 100K [23,25,32]. Usually, the data from magnetic measurements are in fair agreement with those determined from o t h e r experiments, however, the data from different laboratories exhibit some discrepancy corresponding to various purities of the species used. Except of anomalies in temperature dependence of the susceptibility or magnetization o t h e r anomalies are observed. For example in DyRh2Si2 below "IN there is another anomaly at 15 K. At this temperature the Rh-sublattice passes through an itinerant electron magnetic transition [33]. Some observed anomalies can be ascribed to the CEF c f f c c t s - e.g. DyNi2Si2 [ 19], whereas others to the valence instability (sec [25]). Thc manganese compounds are a distinct g r o u p because in contrary to other the Mn-sublattiee orders magnetically whereas sublattices of other d electrons metals do not. Moreover, they order at temperatures which are frequently an o r d e r of magnitude higher than those of the RE-sublattice [20, 25, 34, 35]. Thus one can observe more than one anomaly in the magnetic characteristics. Moreover some of the Mn compounds exhibit ferromagnetic properties e.g. 1.aMn2Gc2, CeMn2Si2 and PrMn2Oe2 below 306. 322 and 334 K, respectively, [34, 35] whilc in ErMn2Si2 the Er-sublattice is ferromagnetic below I()K [25, 34, 36]. For DyMn2Si2 magnetomctric data indicate a ferromagnetic transition (in the Mnsublattice) but Mossbauer studies suggest an alignment of magnetic moments along the c-axis with the sequence + + + - - - for the Dy-sublatrice [19]. Generally, the observed values of thc magnetic moments are lower than those for high spin configuration and this fact proves the electron transfer of the 4f electron to the 3d shell [37]. Szytula and Siek [37] obscrved that the values of ordering temperature decrease with an increase of the c lattice constant. This fact suggests that the magnetic interactions are probably carried via two X atoms (superexchangc). The authors of [37] claim that the type of magnetic ordering depends strongly on the M n - M n dis-
tances within the basal plane. An antiferromagnetie ordering appears for the M n - M n separation < 2 . g 6 5 ,~, whereas above this value ferromagnetic ordering is observed. Based on the measurements performed in strong magnetic fields, large magnetocrystalline anisotropy for DyMn2Ge2 has been revealed. In NdCo2Ge2 at the field of ~ 2.4 T the spin-flop phase transition was detected, but for PrFe2Ge, two phase transitions are observed. The first at H :~ 0.1T and the second one at H '~ 1.1)T [3g]. Concerning the actinide compounds, except those mentioned above, which do not order magnetically, the others are antiferromagncts and only NpCu~Si2 is ferromagnetic bclow 41 K [39]. The magnetic susceptibilities of NpFc~Si, and NpCo2Si, [40] do not follow Curie--Weiss law. The Mossbauer studies under high pressurc in NpCo2Si2 shows a linear correlation between T~ 2 and isomer shift behavior, which follows from the R K K Y rigid-spin model [41]. In the temperature dependence of the uranium compounds numerous anomalies corresponding to the magnetic phase transitions are observed [30, 42, 43]. Thorium compounds arc antiferromagnetic [44]. A m o n g the I'h('r2Si, type compounds, six groups of magnetic structure have been detected. Five of them are collinear and one contains noncollinear structures. Using one-dimensional Ising Hamiltonian and taking into account only the first (d~-in-planc) and the second (.12-interplane) exchangc interactions Szytuta ct al. [451 gave stability conditions for various types of magnetic structures. For I J 2 / J t l < 1/4 eithcr ferromagnetic or antiferromagnetic ordering of AF! type ( + - + - ) with J~ positive or negative, respectively, can exist. A modulated magnetic ordering with a wave vector k given by cos zrk = - J i / 4 J 2 is stablc if IJ2/J,I > I/4 and the AF I1 type structure ( + - - + ) is stable if -J2/IJ,I ~ I/2. This simple approach gives the magnetic structures with a ferromagnetic coupling in the [001] planes and various couplings bctween planes. However, CcRh2Si2 [23], T b N i 2 S i 2 [24] (k = 1/2, I/2, 0), T b C u z O e , _ and Ho('u2Ge2 [461 (k = !/2, 0, 1/2) are exceptions. Quezcl el al. [23] introduced three exchange interactions: two
w. Suski / Properties o[ the BaAIz and ThMnr2 type derivatives
in-plane and one interplane. Then obtained stability conditions describe the magnetic structures of above mentioned compounds too. In spite of the fact that the AFI type structure is the simple one, some complexity also appears. Namely, in some cases the magnetic moment is tilted from the c-axis direction with an angle 3'. For example in NpCo2Sid2 3' = (52 + 15)0 [39], in HoRhzSiz 3' = (28 + 3)q~ [32] and in ErCo2Ge2 magnetic moment direction is perpendicular to the c-axis [38]. Uranium compounds exhibit particularly complicated magnetic structures of the LSW (Iongitudional spin wave) type which exist in limited part of the ordered range and can transform to collinear form. Very interesting behavior is observed for UPd2Si2, in which the magnetic moment of the U ion exhibits two Fourier components from which one becomes zero at 40 K [30]. From the crystallographic point of view the ThMn~2 derivatives are more complicated than previously described ternaries. In the contrary to the ThCr2Si2 type compounds it seems that al least in the actinide compounds the M-sublattice is mainly responsible for magnetic properties. The data for ThFe4AI8 [47] very close to those observed for UFeaAi8 [48] provide clear evidence for this statement. Moreover, NpCu,,AI8 and one of two NpCr4AI8 phases are nonmagnetic [31]. In the case of rare earth compounds the magnetic properties are very sensitive to the relative population of iron in various sublattices [2]. Thus magnetic properties are extremely sensitive to a thermal treatment. For example according to [2], the REFe4AI8 compounds exhibit two independent phase transitions. The higher one, at 130K is due to a canted antiferromagnetic phase transition of the iron sublattice, whereas the lower one at ~ 20 K is due to a complicated magnetic ordering in the RE sublattice. However, Sch~ifer and Will [49] claim that there is only one magnetic transition in the Dy and Ho compounds and at 25 K the iron sublattice sets-up a conical spiral structure. But in Er and Tb compounds the iron sublattice is ordered at ~ 100K while that of the RE at -~ 50 K [50]. As mentioned above these compounds are supposed to form superstructure in
199
which the Fe atoms occupy only one (f) site, but recent M6ssbauer studies [51] can be explained only assuming some occupancy of other positions by the Fe atoms. In REFe6AI6 and in REFesAI7 only one ferromagnetic phase transition is observed and this difference is mainly due to the strongly magnetic Fe (j) sublattice being coupled to both the RE and Fe (f) sublattices [2]. In the first type, the ferromagnetic RE sublattice is antiparallel to the ferromagnetic Fe (j) sublattice and to the ferromagnetic component of the canted antiferromagnetic (f) Fe sublattice. However, in REM6AI6 (M = Cr, Mn, Cu and Rh) [52] the magnetization measurements show weak R E - R E antiferromagnetic interactions and strong crystal field effects. Due to the presence of Mn and Cr local moments, the REMn6Ai6 and RECr6AI6 compounds order ferrimagnetically. UFe6AI6 exhibits ferromagnetic character below - 350 K [47, 51]. Perhaps the most exciting systems are REFesAI7 [2, 52] which show numerous unusual properties, namely strong magnetic and thermal hysteresis and time dependence of the magnetization curves. Moreover the temperature dependences of magnetization of the Sm, Tb and Lu compounds show a negative value of magnetization when sample is quickly cooled down in a relatively weak magnetic field. The hysteresis curves of the systems with Yb, Lu and Y are also exceptional because the virgin curves extend to a region beyond that which is covered by the hysteresis loop [2]. In these materials the M6ssbauer studies of S7Fe reveal at least three magnetically inequivalent sites [53] and the authors suggest that the spin structure is similar to that proposed for REFe6AI6. This scheme of spin structure together with a strong anisotropy (exchange and CEF) can explain most of unusual features of these materials in terms of simple molecular field model [2]. UFesAI7 exhibits less complicated behavior. The temperature dependence of magnetization exhibits an anomaly at about 200-240 K most probably corresponding to the ordering temperature. The temperature is close to those observed for the RE compounds but even at room temperatur the magnetization versus magnetic field curve preserves some curvature [47].
2()()
W. 5uskt / Propertte,~ of the B a A I , and IhMn~ . tYt)e dertt'attt'es
Acknowledgements The a u t h o r is grateful to Profs. A Szvtula and A. Z y g m u n t for v a l u a b l e discussions.
References [I] J. I,eciejewicz, in: ('rystalline Electric Field Effects m f-electron Magnetism, R.P. (;uertin. W. Suski and Z 2",olnierck, eds. (Plenum. New' York. 1982). p. 27t). 12] [. Nowik and I. Felner. in: Proc. Intern. Conf. tm Magnetism of Rare Earths and Actinides. Bucharest 1983, p. 24. 131 K.R. Andress and E. Albcrti, Z. Metallkd, 27 ( 1935t 12. [4] Z. Ban and M. Sikirica, Acta ('ryst. 18 (1965) 594. [5] O.S. Zarechnyuk, P.I. Kripyakcvich and I','.I. (ilad~,shevskii, Soy. Phys. ('rystallogr. tl (1965) 7116. [6] E. Parthe, B. (_'hal~t. H.F. Braun and N. Engel, Acta ( ' r y s t . B39 (19831 588. [7] W.B. P e a r s o n , J. l , e s s - ( ' o m m o n Metals 97 l l9841 I1% 133, 141. [8] ('. I l o r v a t h and P. R o g l , Mat. Rcs. Bull. 18 (1983) 443. [i)] H.F. B r a u n , N. E n g e l and E. Parthd, Phys. Rev. B28 (1983) 1389. [ IOl A. Zygmunt, in: Prt~:. 1 l l m m s Jl)urndcs des Actinides. Jesl)lo 1981. (;. Bombieri, G. de l)aoh~ and P. Zanella, eds., p. 122. { ! 11 W.B. P e a r s o n , J. I , e s s - ( ' o m m o n Metals t)6, 11984) 103. [12] J. Leciejewic,', H. Ptasiewicz-B~k and A. Z y g m u n t . phys. Stilt. sol. (a) 54 (1979) K72. [13"1 J. St~piefi-Damm, A. B a r a n and W. Suski, J. I.cssC o m m o n Metals, 102 ( 1 9 8 4 ) L5. [14] J.P. Markowski, A. Baran and W. Suski. unpublished results. 115] V.V. N e m o s h k a l e n k o , \,'. Ya. Nagornyi, B.P. M a m k o . P K . Nikolyuk, P.V. Gel, R.V. L u c i v and M.D. K o t c r l m , Ukr. Fiz. Zh. 2 6 1 1 9 8 1 ) 1831. 116] R.D. P a r k s , B. Reihl, N. M a r t e n s o n and F. Stcglich, Phys. R e v . . B27 11983) 61)52. [171 ('. G o d a r t , L . ( ' . G u p t a and M.F. Ravet-Krill. J. l.essC o m m o n Metals 94 (19831 187. [ 181 W. Schlabitz, J. Baumann, G. N e u m a n n , [). P l u m a c h e r and K. R e g g e n t m , in: ('rystalline Electric I:ield Effects in f-clectron M a g n e t i s m , R.P. ( i u c r t i n . W. Sttskl and ,~ Zolnierek, eds. (Plenum, New Y o r k , 1982), p. 2St). [19] E. G 0 r l i c h . R. K m i e d , B. J a n u s and A. Szytula, ibid. p. 3(11. 120] N. R u s s m a n n , HLI. I l a f n c r and I). WohlleN.,n, ibid. p. 333. [21] G . A . S t e w a r t and J. Z u k r o w s k i , ibid. p. 3 1 9 [22] S. H o r n , E. Holland-Moritz, M. L o c w e n h a u p t , I I Schcncr, A. Benoit and J. Floquet, Phys. Rev. 1'123 11081) 3171.
i23] S. Ouczel..I. Rossat-Mignod, B. ('he~, alicr, P. l.eja~, and J. l-toruneau, Sol. State ('ommun. 49 (1984) 685. i24] \ ' . N . Nguyen. F. lche,m, J. Rossat-Mignod and A . R Ballestracci, Solid Slate ('ommun. 45 (1983) 2 0 9 [25J .I.I.eciejewic/, M..~l~ski and A. Szvtula. Proe Intern ('onf. on Magnetism of Rare Farths and Aetinide4, Bucharest 1983, p. 108. ]26] B. Sales and D. Wohllebcn. Ph,,,s. Re,. l.eth:rs 35 !l~t?s~ 1240. [271 E. Holland-Morilz, D. Wohlleben and M. l,t~wcnhaupt. Phys. Re',.. B25 11982) 7482 12HI K. Itiebl, ('. I t o r v a l h , I). Rogl and M.J. Sienkt~. Solid State ('ommun. 4~ 119831 21 I. 1291 K. Hicbl, ('. Horvath. P. Rogl and M.J. Sicnko. J. Magn Magn. Malt. 37 (19831 287 130] H. Ptasiewicz-B+k, .I.l.ecielcwicz and A. Zygmunl, I Ph,,s F: Metal Phys. II llt~Sl) 1225. [311 J. (ial. M. Kuznietz. S. Fredro. M. Melamud, H. Pinto. II. Shaken, W. Potzcl, I,. Asch and ( i . M . Kalvius. Proc. 13crees Journces des Actinides. Elat 1983, ('3. 1321 M. ,Sl+ski. J. l,eciejewicz and A. Szytula, J. Mash. Magn Mitt 30 ( 1 0 8 3 ) 20,8 1331 1. Fclner, I N o w i k and M. Sch. P r o c . Intern. C o n f . on M a g n e t i s m ol Rare E a r t h s and Actinides, Bucharest I t)N3. p I 12. 134] A. Szvlula and 1. S,'ott, Solid State (.'ommun. 41) lit)Nil It)t) [35] K.S.V.I,. N a r a s i m h a n , \".1 I.S. Rat), W.E'. Wallace and I Pop, AlP ( ' o n f . P r o c . 29 119751 194. 13~1 ,I. Lccicjcwicz. S. Sick and A. Sz',tuta, I Magn. Magn Mat. 411 I 1~184) 2¢~5 1371 A. Szytuta and S. Sick, J. Magn. M a s s . Mat. 27 i 11~821 49.
1381 J. l,ecicjewicz, A. Szytula and A. Zygmunt, Solid State ( ' o m m u n . 45 ( 1 0 8 3 ) 1 4 0 13~)1 ('.1t. tie Novilm, J. (ial and ,I. BuevoT, J. M a s s . M a s s . Malt. 2 I (It)gO) 85. 1401 J. ( ; a l , M. K r o u p , Z. t t a d a r i and I. Nowik, Phi,,< Rcv. 1:116 (1077) 3013. [41 ] W. Potzel, J. M o s c r , ( i . M . Kalvius, ( ' . H . de N o v i o n , ,I.C Spirlet and J. (}al, Phys. Rev. B24 ( 1 0 8 1 ) 6762. 1421 1.. ( ' h e | m i c k i , J. I.eciejewicz and A Zygmunt, Solid State C o m m u n . 48 (1983! 177. [43] .I Leciejewicz. I,. ( ' h e l m i c k i and A Zygrnunl, Solid State ( ' o m m u n . 41 (1982) 167. [44J Z. Ban, I,. ( ) m e j e c , A. Szytula and Z. l ' o m k o w i c z , ph}s. slat. sol. (a) 27 (1975) 333. [451 A. Szytula, W. B a i e l a and .I. l,cciejewic.< Solid Static C o m m u n . 48 ( 1 9 8 3 ) 1053. [46] P. Sehobinger-Papamantelh)s, A. Niggli. P . A Kotsanidis and J.K. Yakinthos, Inst. f. R e a k t o r t e c h n i k F T l l . Progress R e p o r t 1983. p. 80. 147] A. I'laran, W. Suski anti I M?,dlar;,, these Proc., I'hv,,ica 13()B119851 2 1 9 148] A. Baran, W. Suski and T. Mvdlarz. J. l , e s s - ( ' o m m o n Metals 96 t 1 9 8 4 ) 2~t~. 14'4] W. S c h a f e r and (i. W i l l . . I I,css-('omrnlm Metals ~4 119831 205.
W. Suski / Properties of the BaAl~ and ThMnl., type derivatives [50] M.O. Bargouth, G. Will and K.H.J. Buschow, J. Magn. Magn. Mat. 6 (1977) 129. [51] M. Lukasiak, A. Baran, Z. Kucharski, W. Suski and J. Suwalski, to be published.
201
[52] I. Felner, M. Seh and l. Nowik, J. Phys. Chem. Solids 12 (1981) 1091. [.g3] l. Fclner, 1. Nowik and M. Seh, J. Magn. Magn. Mat. 38 (1983) 172.
S T R U C T U R E , M A G N E T I C A N D R E L A T E D PROPERTIES OF THE T E R N A R Y B a A i 4 A N D ThMnlz-TYPE C O M P O U N D S Wojciech SUSKI Institute for Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 5 0 - 9 5 0 Wroclaw 2, Poland Invited paper In this review the crystal structure of tetragonal, ternary BaAI4 and ThMn~2 derivatives (14/mmm) is presented. The T h C r : S i , as well as (RE, An)M,AI,, (RE, An)MsAI7 and (RE, An)M~AI~ compounds deserve particular attention. The problem of bonding in these materials is discussed in terms of a possible heterodesmic character. Then the investigations of the crystal field, are briefly reported. Finally, magnetic properties including magnetic structures and Mossbauer eftect examinations are described.
1. Introduction The structure, magnetic and related properties of the tetragonal ternaries with the BaAI4 and ThMn~2 type derivative structure (I4/mmm) arc presented below. These types of structure are represented among numerous compounds of the rare earths and actinides. Particular attention deserved the ThCrzSi2-type ternaries (BaAI4 derivatives) because of their various exciting properties like heavy fermion superconductivity, dense Kondo behavior, valence instabilities and interesting magnetic properties found in these materials. Also the ThMnlz-type derivatives, especially these of the rare earths, exhibit fascinating magnetic properties. Therefore an extensive presentation of these materials seems to be interesting. Both groups of compounds have the same body centered tetragonal structure (14/mmm) and have been already reviewed by Leciejewicz [1] and Nowik and Felner [2]. Thus at present we try complete those papers emphasizing some of their highlights.
2. Crystal structure The tetragonai BaAl4-type structure was originally determined by Andress and Alberti [3] but the first ternary derivative which will be at most a
subject of the present paper - ThCrzSi2 was reported by Ban and Sikirica [4] and independently by Zarechnyuk et al. (for CaAI2Gez) [5]. Recently, this type of structure along with its derivatives were discussed in details by Parth6 et al. [6] and Pearson [7]. More than 200 rare earth (RE) or actinide (An) transition metal (M) borides, gailides, silicides, germanides and phosphides (X) (see [1, 6]) crystallize with the ThCr2Siz-type of structure. The space group is 14/mmm and the c/a ratio in these compounds is usually close to 2.5. The Th atoms are surrounded by 22 atoms (four Th atoms, eight Cr atoms and eight plus two Si atoms) in a polyhedron with a surface composed of 16 triangular and 12 square faces. In this structure eight 4(RE,An)-M, eight I(RE,An)-X and 1X-X interatomic contacts are generally close enough to be of interest in bonding and magnetic interaction problem, whereas other contacts are generally at distances that are so much greater than the appropriate radius sums, as to appear to be of no interest. The similar conclusion concerning a very weak C e - C e bonding (if any) was presented in [8] and this statement was extended to a new series of isotypic compounds {RE, Th, U)(Ru,Os)2Si2. In discussion on interatomic contacts as the cell dimension controlling factors, Pearson [7] observed that the behavior of the Mn-alloys generally differs from that of the
0378-4363/85/$03.30 (c] Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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W. Suski / Properties of the HaAl~ anti ThMnt., type derwattve~
o t h e r transition metals. l"his r e m a r k is i m p o r t a n t in relation to f u r t h e r discussion on m a g n e t i c properties of these materials. Recently, B r a u n ct al. [9] r e p o r t e d p o l y m o r p h i s m of l,alr2Si2 comp o u n d exhibiting a high t e m p e r a t u r e form crystallizing with the primitive, tetragonal C a B c , G c , type of structure ( P 4 / n m m ) . ( ' e r l a i n l v lhe l ' h M n ~ e type of structure and its derivatives are less p o p u l a r than the c o m p o u n d s discussed a b o v e . The most p o p u l a r derivatives arc (RE,An)M,AI,,. I RE,An)MsAI7 and (RE,An)M4AIx. In this b o d y - c e n t e r e d tetragonal crystal structure (14/mmm) the R E / A n a t o m s o c c u p y the 2 (a) sites. In (RE,An)M~,AI,, the (f) positions arc o c c u p i e d by M a t o m s and the (il positions by AI atoms, w h e r e a s the r e m a i n i n g M and AI are distributed at r a n d o m over the (j) sites [2]. The 57Fe M o s s b a u e r studies of REFe~AI7 [2J show that 5 2 - 6 0 % of the iron a t o m s o c c u p y the (f) sites, a b o u t 4(1% the (j) sites and 11-10% the (i) sites. The (RE,AnlM4AI~ c o m p o u n d s form a superstructure of the CeMn4Al~-type in w h i c h M and AI a t o m s are not distributed statistically over the three a t o m i c positions available, but instead the M a t o m s o c c u p y only one (f) site while the AI a t o m s two o t h e r ones ((i) and (j)). Pearson [11] p o i n t e d out that the cell d i m e n s i o n s of the ( R E , A n t M 6 A I , and ( R E , A n l M 4 A I g phases ind i c a t e d relative transition m e t a l sizes in the o r d e r Fe < Cu < Mn < Cr, w h e r e a s in the e l e m e n t a l m e t a l s m a n g a n e s e is larger than c h r o m i u m w h i c h is approximately of the same size as iron. In the e l e m e n t a l m e t a l s the relative sizes u n d o u b t l v result from m a g n e t i c interactions but in the ternaries the o r d e r of sizes suggests an a b s e n c e of s t r o n g m a g n e t i c interactions. This s t a t e m e n t s e e m s to be not entirely valid in the light of the m a g n e t i c properties of these alloys described below.
3. Chemical bonding The p r o b l e m of c h e m i c a l Ix)nding in the tetragonal u r a n i u m ternaries, including the "l'hCr2Si2 type phases has been considered bv Z y g m u t [l(l]. It s e e m s that his main conclusion.,, can be also applied to the rare e a r t h c o m p o u n d s .
For the quantitative discussion hc selected EJPd2Si2 for w h i c h the crystal structure has been precisely refined 1112]. In c o o r d i n a t i o n polyh e d r o n of uranium [Si(UaPd4Si)] onc can distinguish the shortest Si-Si, S i - P d , Si-U and t l-Pd distances, which are e q u a l to 2.38, 2.44 3.13 and 3.24 ,g,, respectively, it is c o n c l u d e d that the lirst and the s e c o n d distances c o r r e s p o n d to sum of the c o v a l e n t radii, and thus to c o v a l e n t b o n d i n g . The Si-U distance from one hand is shorter than simple sum of i o n i c radii 13.68 ]k) but l o n g e r than the sum of c o v a l e n t radii (2.59 /~). Because the Si-atoms is strongly electronegative with respect to u r a n i u m , one can e x p e c t the i o n i c - c o v a l e n t b o n d i n g . Finally, the P d - P d and U - P d b o n d i n g s are c o n s i d e r e d as metallic. T h e r e f o r e , one can a s s u m e heterodesmic c h a r a c t e r of chemical b o n d i n g to exist in the ThCr2Si2 structure. For UFeaAI~ the shortest distances a m o u n t to [13] u - u = 5 . 0 3 6 ,~, U - F c = 3.34O ~ , U-AI = 3 . 0 0 9 ~ , F e - F e = 2.518 A, F e - A I = 2 . 5 3 8 /~ and AIA 1 = 2 . 7 3 0 ~ , but t a k i n g into a c c o u n t the c h a r a c t e r of e l e m e n t s and high electrical conductivity of these materials [14] one can expect metallic c h a r a c t e r of b o n d i n g in the ThM,.3 dcrivativcs. N c m o s h k a l e n k o ct al. [15] suggest also a heterodesmic c h a r a c t e r of bonding {covalent and m e t a l l i c ) i n the REAI2Si2 type family, which in spite of the same stoichiomctry is representative of the hexagonal La2(),S type of structure. For ( ' e ' l ' 2 S i 2 l T = ( ' u , Ag, Au, Pdl c o m p o u n d s it is the hybridization of Cc 4f e l e c t r o n s with "l'-dcrivcd d-states w h i c h is supposed to d e t e r m i n e the g r o u n d state properties (superconductivity for T = ('u. o r d e r e d m a g n e t i s m for 1" = Ag, Au, Pdl. This s t a t e m e n t was c o n c l u d e d from the r e s o n a n t photoemission studies on these systems [ 16]. It was o b s e r v e d that for RERh2Si2 series lanthanide c o n t r a c t i o n manifests itself in the c-lattice c o n s t a n t , w h e r e a s in REPd2Si2 in the a-lattice c o n s t a n t suggesting a fine difference in bonding c h a r a c t e r in these two directions [171.
4. Crystal field This p r o b l e m was not considered up to now in details. No suggestion was published c o n c e r n i n g
W. Suski / Properties of the BaAIa and ThMnl. type derivatives
the ThCrzSi2 type c o m p o u n d s of actinides and the ThMn~2 derivatives from obvious reason of complexity of these materials. For the q'hCr2Si2 type c o m p o u n d s of rare earths no information is available with respect to d electron metals. According to neutron diffraction results d electrons in these materials, except of Mn-compounds, are itinerant and thus it is difficult to discuss thc crystal field ( C E F ) problem. Because the values of rare e a r t h magnetic m o m e n t s are as a rule very close to the Ixng,,/J(J + 1) value, one can conelude that the C E F acting on the rare e a r t h ion is weak. The C E F Hamiltonian for a rare e a r t h ion in a tetragonal point symmetry is given by =
B404
+ B ] ( ) ~ + B . O ~ , + l::J,,06,
but as mentioned above, the general discussion of magnitude of the individual terms and corresponding level schemes is lacking. In individual cases the consideration of magnetic and related properties in terms of the C E F is more detailed. Schlabitz et al. [18] claim that a slight deviation of the magnetic susceptibility of RECu2Si2 (RE = Th, Dy, Ho, Er and Tm) and DyNizSi2 [19] from the C u r i e - W e i s s law below 100 K, is apparently due to the C E F effects. Moreover, they report [18] magnetostriction data of (Y~_~RE~)CuzSi2 which follow the Stevens f a c t o r in sign and to some extent even in magnitude across the RE series, except for Tm, Nd and Sm [20]. Gorlich et al. [19] however, are convinced that the C E F interactions effect considerably the nuclear hyperfine coupling in the dysprosium compounds. Stewart and Zukrowski [21] try to explain their 1 6 9 T m Mossbauer data for TmCuzSi2 in t e r m s of the C E F effects but using the C E F parameters obtained before for CeCuzSi2 by m e a n s of neutron diffraction [22]. For TmCu2Si~ they assume the C E F level scheme with 2 low-lying singlets well separated from the three o t h e r singlets and four n o n - K r a m e r s doublets. This s c h e m e gives a fair fit of experimental results. For CeRh2Si2 the following functions were proposed for the C E F ground-state doublet: 10) = 0.91 + 5/2) + 0.18] + 3/2) [22]. At the same time the large values of the magnetic m o m e n t found in TbNi2Si2 and TbCozSi2 [24], TbRu2Si2
197
[25] and TbRhzSi2 [23] suggest that the C E F of D4h symmetry acting on the T b 3. ion has to p r o d u c e a g r o u n d state with two very close singlets: 1/,/21161 +1 - 6)). In this case the magnetic m o m e n t s are quenched a l o n g the c-axis giving rise to an Ising-like behavior. Some authors claim that the magnetic structure of c o m p o u n d depends on the sign of the B ° term (see [19,211]). In accordance with the previous conclusion that the crystal field in the ThCrzSi2 c o m p o u n d s is weak, an overall splitting in CeCu2Si2 of 31 meV was found by inelastic neutron scattering [22]. Moreover, the anisotropy of the static susceptibility of the YbCu2Si2 single crystals [26] shows that the C E F effects must o v e r c o m e the valence fluctuations. However, the results obtained for the Ce and Yb c o m p o u n d s could be exceptional because the ('e ion has at most 4fI electron, whereas the Yb ion the 4f-~ hole. For YbCu2Si2 in a tetragonal crystal field the J = 7/2 H u n d ' s rule g r o u n d state splits into four doublets. The careful analysis [27] of paramagnetic neutron spectra yields two most likely sets of the C E F parameters namely: 1) W = 2 . 5 + 0 . 3 meV, x~ =-11.41 + 0 . 1 , x2 = + 0 . 0 7 + 1 1 . 0 5 , x3=(1.15+11.115 and x4=(1.13 ± 0 . 0 5 or 2. W = -2.5 +0.3 meV, x, = 0.4 + 0 . 1 , x2 = ± 0.15 + 0.05, x3 = -(1.()7 :i: 0.05, x4 = + 11.13 + 0.05. Both sets have similar energy-level sequence of 11--18-25-31 meV.
5. Magnetic properties A c c o r d i n g to the author's best knowledge, magnetic properties (including magnetic structure and M6ssbauer effect) of more than 120 c o m p o u n d s of rare earths with the ThCr2Si2 type of structure have been examined. A m o n g them about 25 a p p e a r e d to be nonmagnetic and 12 superconducting. At the same time a m o n g 12 actinide compounds (with Th, U and Np) UOszSi2 [28], URu2Si2 [29] and URh2Ge2 [311] exhibit paramagnetic properties. All the ThMn12 type compounds both of the rare earths and actinides, except of NpCu4AIs and one modification of NpCr4AIs, are reported to be magnetically o r d e r e d [31]. As a rule vast
It~8
w. Suski / Properties of the Ba/'iLj and lhMn,., type derit'atires
majority of the ThCr2Si2 type compounds exhibits antiferromagnetic properties with the N6el point below 50 K (see eg. [I]). ()nly for somc terbium compouds the N6el point is close to 100K [23,25,32]. Usually, the data from magnetic measurements are in fair agreement with those determined from o t h e r experiments, however, the data from different laboratories exhibit some discrepancy corresponding to various purities of the species used. Except of anomalies in temperature dependence of the susceptibility or magnetization o t h e r anomalies are observed. For example in DyRh2Si2 below "IN there is another anomaly at 15 K. At this temperature the Rh-sublattice passes through an itinerant electron magnetic transition [33]. Some observed anomalies can be ascribed to the CEF c f f c c t s - e.g. DyNi2Si2 [ 19], whereas others to the valence instability (sec [25]). Thc manganese compounds are a distinct g r o u p because in contrary to other the Mn-sublattiee orders magnetically whereas sublattices of other d electrons metals do not. Moreover, they order at temperatures which are frequently an o r d e r of magnitude higher than those of the RE-sublattice [20, 25, 34, 35]. Thus one can observe more than one anomaly in the magnetic characteristics. Moreover some of the Mn compounds exhibit ferromagnetic properties e.g. 1.aMn2Gc2, CeMn2Si2 and PrMn2Oe2 below 306. 322 and 334 K, respectively, [34, 35] whilc in ErMn2Si2 the Er-sublattice is ferromagnetic below I()K [25, 34, 36]. For DyMn2Si2 magnetomctric data indicate a ferromagnetic transition (in the Mnsublattice) but Mossbauer studies suggest an alignment of magnetic moments along the c-axis with the sequence + + + - - - for the Dy-sublatrice [19]. Generally, the observed values of thc magnetic moments are lower than those for high spin configuration and this fact proves the electron transfer of the 4f electron to the 3d shell [37]. Szytula and Siek [37] obscrved that the values of ordering temperature decrease with an increase of the c lattice constant. This fact suggests that the magnetic interactions are probably carried via two X atoms (superexchangc). The authors of [37] claim that the type of magnetic ordering depends strongly on the M n - M n dis-
tances within the basal plane. An antiferromagnetie ordering appears for the M n - M n separation < 2 . g 6 5 ,~, whereas above this value ferromagnetic ordering is observed. Based on the measurements performed in strong magnetic fields, large magnetocrystalline anisotropy for DyMn2Ge2 has been revealed. In NdCo2Ge2 at the field of ~ 2.4 T the spin-flop phase transition was detected, but for PrFe2Ge, two phase transitions are observed. The first at H :~ 0.1T and the second one at H '~ 1.1)T [3g]. Concerning the actinide compounds, except those mentioned above, which do not order magnetically, the others are antiferromagncts and only NpCu~Si2 is ferromagnetic bclow 41 K [39]. The magnetic susceptibilities of NpFc~Si, and NpCo2Si, [40] do not follow Curie--Weiss law. The Mossbauer studies under high pressurc in NpCo2Si2 shows a linear correlation between T~ 2 and isomer shift behavior, which follows from the R K K Y rigid-spin model [41]. In the temperature dependence of the uranium compounds numerous anomalies corresponding to the magnetic phase transitions are observed [30, 42, 43]. Thorium compounds arc antiferromagnetic [44]. A m o n g the I'h('r2Si, type compounds, six groups of magnetic structure have been detected. Five of them are collinear and one contains noncollinear structures. Using one-dimensional Ising Hamiltonian and taking into account only the first (d~-in-planc) and the second (.12-interplane) exchangc interactions Szytuta ct al. [451 gave stability conditions for various types of magnetic structures. For I J 2 / J t l < 1/4 eithcr ferromagnetic or antiferromagnetic ordering of AF! type ( + - + - ) with J~ positive or negative, respectively, can exist. A modulated magnetic ordering with a wave vector k given by cos zrk = - J i / 4 J 2 is stablc if IJ2/J,I > I/4 and the AF I1 type structure ( + - - + ) is stable if -J2/IJ,I ~ I/2. This simple approach gives the magnetic structures with a ferromagnetic coupling in the [001] planes and various couplings bctween planes. However, CcRh2Si2 [23], T b N i 2 S i 2 [24] (k = 1/2, I/2, 0), T b C u z O e , _ and Ho('u2Ge2 [461 (k = !/2, 0, 1/2) are exceptions. Quezcl el al. [23] introduced three exchange interactions: two
w. Suski / Properties o[ the BaAIz and ThMnr2 type derivatives
in-plane and one interplane. Then obtained stability conditions describe the magnetic structures of above mentioned compounds too. In spite of the fact that the AFI type structure is the simple one, some complexity also appears. Namely, in some cases the magnetic moment is tilted from the c-axis direction with an angle 3'. For example in NpCo2Sid2 3' = (52 + 15)0 [39], in HoRhzSiz 3' = (28 + 3)q~ [32] and in ErCo2Ge2 magnetic moment direction is perpendicular to the c-axis [38]. Uranium compounds exhibit particularly complicated magnetic structures of the LSW (Iongitudional spin wave) type which exist in limited part of the ordered range and can transform to collinear form. Very interesting behavior is observed for UPd2Si2, in which the magnetic moment of the U ion exhibits two Fourier components from which one becomes zero at 40 K [30]. From the crystallographic point of view the ThMn~2 derivatives are more complicated than previously described ternaries. In the contrary to the ThCr2Si2 type compounds it seems that al least in the actinide compounds the M-sublattice is mainly responsible for magnetic properties. The data for ThFe4AI8 [47] very close to those observed for UFeaAi8 [48] provide clear evidence for this statement. Moreover, NpCu,,AI8 and one of two NpCr4AI8 phases are nonmagnetic [31]. In the case of rare earth compounds the magnetic properties are very sensitive to the relative population of iron in various sublattices [2]. Thus magnetic properties are extremely sensitive to a thermal treatment. For example according to [2], the REFe4AI8 compounds exhibit two independent phase transitions. The higher one, at 130K is due to a canted antiferromagnetic phase transition of the iron sublattice, whereas the lower one at ~ 20 K is due to a complicated magnetic ordering in the RE sublattice. However, Sch~ifer and Will [49] claim that there is only one magnetic transition in the Dy and Ho compounds and at 25 K the iron sublattice sets-up a conical spiral structure. But in Er and Tb compounds the iron sublattice is ordered at ~ 100K while that of the RE at -~ 50 K [50]. As mentioned above these compounds are supposed to form superstructure in
199
which the Fe atoms occupy only one (f) site, but recent M6ssbauer studies [51] can be explained only assuming some occupancy of other positions by the Fe atoms. In REFe6AI6 and in REFesAI7 only one ferromagnetic phase transition is observed and this difference is mainly due to the strongly magnetic Fe (j) sublattice being coupled to both the RE and Fe (f) sublattices [2]. In the first type, the ferromagnetic RE sublattice is antiparallel to the ferromagnetic Fe (j) sublattice and to the ferromagnetic component of the canted antiferromagnetic (f) Fe sublattice. However, in REM6AI6 (M = Cr, Mn, Cu and Rh) [52] the magnetization measurements show weak R E - R E antiferromagnetic interactions and strong crystal field effects. Due to the presence of Mn and Cr local moments, the REMn6Ai6 and RECr6AI6 compounds order ferrimagnetically. UFe6AI6 exhibits ferromagnetic character below - 350 K [47, 51]. Perhaps the most exciting systems are REFesAI7 [2, 52] which show numerous unusual properties, namely strong magnetic and thermal hysteresis and time dependence of the magnetization curves. Moreover the temperature dependences of magnetization of the Sm, Tb and Lu compounds show a negative value of magnetization when sample is quickly cooled down in a relatively weak magnetic field. The hysteresis curves of the systems with Yb, Lu and Y are also exceptional because the virgin curves extend to a region beyond that which is covered by the hysteresis loop [2]. In these materials the M6ssbauer studies of S7Fe reveal at least three magnetically inequivalent sites [53] and the authors suggest that the spin structure is similar to that proposed for REFe6AI6. This scheme of spin structure together with a strong anisotropy (exchange and CEF) can explain most of unusual features of these materials in terms of simple molecular field model [2]. UFesAI7 exhibits less complicated behavior. The temperature dependence of magnetization exhibits an anomaly at about 200-240 K most probably corresponding to the ordering temperature. The temperature is close to those observed for the RE compounds but even at room temperatur the magnetization versus magnetic field curve preserves some curvature [47].
2()()
W. 5uskt / Propertte,~ of the B a A I , and IhMn~ . tYt)e dertt'attt'es
Acknowledgements The a u t h o r is grateful to Profs. A Szvtula and A. Z y g m u n t for v a l u a b l e discussions.
References [I] J. I,eciejewicz, in: ('rystalline Electric Field Effects m f-electron Magnetism, R.P. (;uertin. W. Suski and Z 2",olnierck, eds. (Plenum. New' York. 1982). p. 27t). 12] [. Nowik and I. Felner. in: Proc. Intern. Conf. tm Magnetism of Rare Earths and Actinides. Bucharest 1983, p. 24. 131 K.R. Andress and E. Albcrti, Z. Metallkd, 27 ( 1935t 12. [4] Z. Ban and M. Sikirica, Acta ('ryst. 18 (1965) 594. [5] O.S. Zarechnyuk, P.I. Kripyakcvich and I','.I. (ilad~,shevskii, Soy. Phys. ('rystallogr. tl (1965) 7116. [6] E. Parthe, B. (_'hal~t. H.F. Braun and N. Engel, Acta ( ' r y s t . B39 (19831 588. [7] W.B. P e a r s o n , J. l , e s s - ( ' o m m o n Metals 97 l l9841 I1% 133, 141. [8] ('. I l o r v a t h and P. R o g l , Mat. Rcs. Bull. 18 (1983) 443. [i)] H.F. B r a u n , N. E n g e l and E. Parthd, Phys. Rev. B28 (1983) 1389. [ IOl A. Zygmunt, in: Prt~:. 1 l l m m s Jl)urndcs des Actinides. Jesl)lo 1981. (;. Bombieri, G. de l)aoh~ and P. Zanella, eds., p. 122. { ! 11 W.B. P e a r s o n , J. I , e s s - ( ' o m m o n Metals t)6, 11984) 103. [12] J. Leciejewic,', H. Ptasiewicz-B~k and A. Z y g m u n t . phys. Stilt. sol. (a) 54 (1979) K72. [13"1 J. St~piefi-Damm, A. B a r a n and W. Suski, J. I.cssC o m m o n Metals, 102 ( 1 9 8 4 ) L5. [14] J.P. Markowski, A. Baran and W. Suski. unpublished results. 115] V.V. N e m o s h k a l e n k o , \,'. Ya. Nagornyi, B.P. M a m k o . P K . Nikolyuk, P.V. Gel, R.V. L u c i v and M.D. K o t c r l m , Ukr. Fiz. Zh. 2 6 1 1 9 8 1 ) 1831. 116] R.D. P a r k s , B. Reihl, N. M a r t e n s o n and F. Stcglich, Phys. R e v . . B27 11983) 61)52. [171 ('. G o d a r t , L . ( ' . G u p t a and M.F. Ravet-Krill. J. l.essC o m m o n Metals 94 (19831 187. [ 181 W. Schlabitz, J. Baumann, G. N e u m a n n , [). P l u m a c h e r and K. R e g g e n t m , in: ('rystalline Electric I:ield Effects in f-clectron M a g n e t i s m , R.P. ( i u c r t i n . W. Sttskl and ,~ Zolnierek, eds. (Plenum, New Y o r k , 1982), p. 2St). [19] E. G 0 r l i c h . R. K m i e d , B. J a n u s and A. Szytula, ibid. p. 3(11. 120] N. R u s s m a n n , HLI. I l a f n c r and I). WohlleN.,n, ibid. p. 333. [21] G . A . S t e w a r t and J. Z u k r o w s k i , ibid. p. 3 1 9 [22] S. H o r n , E. Holland-Moritz, M. L o c w e n h a u p t , I I Schcncr, A. Benoit and J. Floquet, Phys. Rev. 1'123 11081) 3171.
i23] S. Ouczel..I. Rossat-Mignod, B. ('he~, alicr, P. l.eja~, and J. l-toruneau, Sol. State ('ommun. 49 (1984) 685. i24] \ ' . N . Nguyen. F. lche,m, J. Rossat-Mignod and A . R Ballestracci, Solid Slate ('ommun. 45 (1983) 2 0 9 [25J .I.I.eciejewic/, M..~l~ski and A. Szvtula. Proe Intern ('onf. on Magnetism of Rare Farths and Aetinide4, Bucharest 1983, p. 108. ]26] B. Sales and D. Wohllebcn. Ph,,,s. Re,. l.eth:rs 35 !l~t?s~ 1240. [271 E. Holland-Morilz, D. Wohlleben and M. l,t~wcnhaupt. Phys. Re',.. B25 11982) 7482 12HI K. Itiebl, ('. I t o r v a l h , I). Rogl and M.J. Sienkt~. Solid State ('ommun. 4~ 119831 21 I. 1291 K. Hicbl, ('. Horvath. P. Rogl and M.J. Sicnko. J. Magn Magn. Malt. 37 (19831 287 130] H. Ptasiewicz-B+k, .I.l.ecielcwicz and A. Zygmunl, I Ph,,s F: Metal Phys. II llt~Sl) 1225. [311 J. (ial. M. Kuznietz. S. Fredro. M. Melamud, H. Pinto. II. Shaken, W. Potzcl, I,. Asch and ( i . M . Kalvius. Proc. 13crees Journces des Actinides. Elat 1983, ('3. 1321 M. ,Sl+ski. J. l,eciejewicz and A. Szytula, J. Mash. Magn Mitt 30 ( 1 0 8 3 ) 20,8 1331 1. Fclner, I N o w i k and M. Sch. P r o c . Intern. C o n f . on M a g n e t i s m ol Rare E a r t h s and Actinides, Bucharest I t)N3. p I 12. 134] A. Szvlula and 1. S,'ott, Solid State (.'ommun. 41) lit)Nil It)t) [35] K.S.V.I,. N a r a s i m h a n , \".1 I.S. Rat), W.E'. Wallace and I Pop, AlP ( ' o n f . P r o c . 29 119751 194. 13~1 ,I. Lccicjcwicz. S. Sick and A. Sz',tuta, I Magn. Magn Mat. 411 I 1~184) 2¢~5 1371 A. Szytuta and S. Sick, J. Magn. M a s s . Mat. 27 i 11~821 49.
1381 J. l,ecicjewicz, A. Szytula and A. Zygmunt, Solid State ( ' o m m u n . 45 ( 1 0 8 3 ) 1 4 0 13~)1 ('.1t. tie Novilm, J. (ial and ,I. BuevoT, J. M a s s . M a s s . Malt. 2 I (It)gO) 85. 1401 J. ( ; a l , M. K r o u p , Z. t t a d a r i and I. Nowik, Phi,,< Rcv. 1:116 (1077) 3013. [41 ] W. Potzel, J. M o s c r , ( i . M . Kalvius, ( ' . H . de N o v i o n , ,I.C Spirlet and J. (}al, Phys. Rev. B24 ( 1 0 8 1 ) 6762. 1421 1.. ( ' h e | m i c k i , J. I.eciejewicz and A Zygmunt, Solid State C o m m u n . 48 (1983! 177. [43] .I Leciejewicz. I,. ( ' h e l m i c k i and A Zygrnunl, Solid State ( ' o m m u n . 41 (1982) 167. [44J Z. Ban, I,. ( ) m e j e c , A. Szytula and Z. l ' o m k o w i c z , ph}s. slat. sol. (a) 27 (1975) 333. [451 A. Szytula, W. B a i e l a and .I. l,cciejewic.< Solid Static C o m m u n . 48 ( 1 9 8 3 ) 1053. [46] P. Sehobinger-Papamantelh)s, A. Niggli. P . A Kotsanidis and J.K. Yakinthos, Inst. f. R e a k t o r t e c h n i k F T l l . Progress R e p o r t 1983. p. 80. 147] A. I'laran, W. Suski anti I M?,dlar;,, these Proc., I'hv,,ica 13()B119851 2 1 9 148] A. Baran, W. Suski and T. Mvdlarz. J. l , e s s - ( ' o m m o n Metals 96 t 1 9 8 4 ) 2~t~. 14'4] W. S c h a f e r and (i. W i l l . . I I,css-('omrnlm Metals ~4 119831 205.
W. Suski / Properties of the BaAl~ and ThMnl., type derivatives [50] M.O. Bargouth, G. Will and K.H.J. Buschow, J. Magn. Magn. Mat. 6 (1977) 129. [51] M. Lukasiak, A. Baran, Z. Kucharski, W. Suski and J. Suwalski, to be published.
201
[52] I. Felner, M. Seh and l. Nowik, J. Phys. Chem. Solids 12 (1981) 1091. [.g3] l. Fclner, 1. Nowik and M. Seh, J. Magn. Magn. Mat. 38 (1983) 172.